Karl:
When kids get little bumps all over, either on the skin or under it (or both), it's usually a virus of some sort. Often there are no other symptoms, such as cough, runny nose, and so on.
If your son is taking no new medications, I would just wait it out for 3-4 more days. treat itch with cool compresses or itch lotions containing Benadryl or pramoxine. Taking oral Benadryl is more trouble than it's worth; kids may get hyper from it instead of less itchy.
If the rash doesn't just fade away, as I predict it will, show it to your pediatrician.
Good luck.
Dr. R